I'm not sure Mac OS 9 USB printer sharing will work with Mac OS X. Also, if it's connected to the iMac G3 via USB, how is AppleTalk part of the process? An AppleTalk printer connects to the network using LocalTalk cables, and is not directly connected to any single computer.
However, if it does work and I'm not getting it, you have to make AppleTalk active on the Leopard side. In System Preferences
Network pane, select whichever network service (AirPort, Built-in Ethernet, etc) you are using to connect to your network in the sidebar. Click the
Advanced button. Go to the
AppleTalk tab. Check the box for +Make AppleTalk Active+.
But again, I don't get how AppleTalk is involved for an USB printer.